Valve gear of engine

ABSTRACT

A valve drive system of an engine includes a camshaft that is supported by a cylinder head of the engine and on which a plurality of cams having different valve lift characteristics are formed at predetermined intervals (pitches), a rocker shaft that is supported by the cylinder head so as to be parallel to the camshaft, and a rocker arm that is swingably supported by the rocker shaft. The rocker arm is arranged between one of the cams and an intake valve or an exhaust valve and is arranged so as to be movable in an axial direction of the rocker shaft. A presser of the rocker arm that presses the intake valve or the exhaust valve extends in the axial direction with a length greater than a formation interval (pitch) between the cams. The valve drive system includes a drive unit that moves the rocker arm toward one side or toward an opposite side in the axial direction by the formation interval between the cams.

TECHNICAL FIELD

The present invention relates to a valve gear or valve drive system ofan engine, and particularly, to a valve drive system that includes aswitching mechanism that performs switching between a plurality of camshaving different valve lift characteristics.

BACKGROUND ART

Conventional techniques for a valve drive system of an engine aredisclosed in, for example, Patent Documents 1 to 3.

A valve drive system disclosed in Patent Document 1 includes a low-speedrocker arm that is pressed by a low-speed cam that is used for a lowspeed, a high-speed rocker arm that is pressed by a high-speed cam thatis used for a high speed, and a switching mechanism that performsswitching between cams to be used. In this valve drive system, an intakevalve or an exhaust valve is connected only to the low-speed rocker arm.

The switching mechanism includes a hydraulic piston that moves betweenthe low-speed rocker arm and the high-speed rocker arm. The hydraulicpiston is stored in the low-speed rocker arm when the low-speed cam isused. The hydraulic piston is engaged both with the low-speed rocker armand with the high-speed rocker arm when the high-speed cam is used.

A valve drive system disclosed in Patent Document 2 includes a switchingmechanism that performs switching between two kinds of cams. Theswitching mechanism includes a roller guide supported by a rocker arm soas to be movable in the axial direction thereof, a roller rotatablysupported by the roller guide, and a cam mechanism that moves the rollerguide in the axial direction. The roller is in contact with either ofthe two kinds of cams. The cam mechanism includes a rail groove and anannular groove that are formed on a camshaft, a follower pin disposed atthe roller guide so as to be able to enter or leave these grooves, and areturn spring that returns the roller guide to an initial position. Aterminal of the rail groove is connected to the annular groove.

In this valve drive system, the roller guide and the roller move towardone side in the axial direction by allowing the follower pin to moveforwardly to be fitted into the rail groove, and one of the two kinds ofcams is connected to the rocker arm. On the other hand, the roller guidereturns to the initial position while receiving a spring force of thereturn spring by allowing the follower pin to move backwardly, and theother one of the two kinds of cams is connected to the rocker arm.

A valve drive system disclosed in Patent Document 3 includes a switchingmechanism that moves two cams having different valve liftcharacteristics in the axial direction of a camshaft. The switchingmechanism includes a cam carrier formed of a cylindrical body that hasthe cams, spiral grooves formed at both ends of the cam carrier, and apair of driving pins that can be inserted into the spiral grooves,respectively. The cam carrier is supported by a main camshaft thatpenetrates the cam carrier. The cam carrier rotates together with themain camshaft, and moves toward one side in the axial direction of themain camshaft by allowing one driving pin to be inserted into one spiralgroove. On the other hand, the cam carrier moves toward the other sidein the axial direction by allowing the other driving pin to be insertedinto the other spiral groove.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Published Examined Patent Application    No. 1-12-43004-   Patent Document 2: Japanese Patent No. 3365805-   Patent Document 3: Japanese Translation of International Application    (Kohyo) No. 2006-520869

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Each of the rocker arms of the valve drive systems disclosed in PatentDocuments 1 and 2 includes the movable member (piston, roller guide) ofthe switching mechanism. Therefore, these valve drive systems increasein the mass of the rocker arm. Additionally, the rocker arm has acomplex structure, and hence has a possibility that a portion of therocker arm may heve a low rigidity. If the rocker arm is great in massand is low in rigidity, a cam action cannot be reliably transmitted tothe intake valve or the exhaust valve during a high-speed operation. Inthis case, the opening/closing timing and the amount of valve lift maybecome inaccurate, thus resulting in damage to the valve drive systemsystem.

Additionally, the valve drive systems disclosed in Patent Documents 1and 2 cannot control the moving speed of the movable members (piston,roller guide). Therefore, the movable members moving at a high speedcollide with a stopper part, and an impact sound occurs.

The high-speed rocker arm of Patent Document 1 is always pressed againstthe high-speed cam by a lost motion spring. The follower pin of PatentDocument 2 is pressed against a side wall of the annular groove by thereturn spring in a state of having moved to the inside of the annulargroove. In other words, in the valve drive systems of Patent Documents 1and 2, there are components that are pressed against a rotating part onthe camshaft side and that are brought into slide contact therewith, andtherefore a loss occurs in engine power.

The cam carrier and the main camshaft of the valve drive system ofPatent Document 3 are splined to each other. Therefore, the connectionstructure formed by the cam carrier and the main camshaft is complex,and production costs are high.

In the valve drive system of any one of Patent Documents 1 to 3, aswitching mechanism is needed for each cam (each cylinder). Therefore,if the valve drive system is used for a multi-cylinder engine, thenumber of switching mechanisms becomes larger correspondingly to a risein the number of cylinders, and production costs become higher.

The present invention has been made to solve or lessen theabove-mentioned problems, and aims to provide a valve drive system of anengine in which the mass of a rocker arm does not become heavy.Additionally, the present invention aims to provide a valve drive systemthat is capable of preventing the occurrence of an impact sound duringswitching and that is capable of reducing a power loss. Stilladditionally, the present invention aims to provide a valve drive systemof an engine that is low in production costs even if the valve drivesystem is used for a multi-cylinder engine.

Means for Solving the Problems

One embodiment of the present invention provides a valve drive system ofan engine, and the valve drive system includes a camshaft that issupported by a cylinder head of the engine and on which a plurality ofcams having different valve lift characteristics are formed at apredetermined formation interval (pitches), a rocker shaft supported bythe cylinder head in parallel with the camshaft, and a rocker arm thatis swingably supported by the rocker shaft. The rocker arm is providedbetween one of the plurality of cams and an intake valve or an exhaustvalve and is arranged so as to be movable in an axial direction of therocker shaft. A presser of the rocker arm with respect to the intakevalve or the exhaust valve extends in the axial direction with a lengthgreater than the formation interval (pitch) between the plurality ofcams. The valve drive system further includes a drive unit that movesthe rocker arm toward one side or toward an opposite side in the axialdirection by the formation interval between the plurality of cams.Preferably, the drive unit is arranged so as to generate a thrust forceto move the rocker arm in the axial direction when the amounts of valvelifts of the plurality of cams are 0 while using a switching cam formedintegrally with the camshaft. Preferably, the drive unit is supported bya part differing from the rocker arm.

The valve drive system has a different structure from those of PatentDocuments 1 to 3, and is arranged so that the rocker arm is movable inthe axial direction of the rocker shaft. The rocker arm is moved by thedrive unit in the axial direction of the rocker shaft. Therefore, thedrive unit can be easily supported by a part other than the rocker arm.By thus disposing the drive unit, a moving component used to performswitching between cams to be used can be disposed at a part differingfrom the rocker arm, and therefore the mass of the rocker arm can bereduced. As a result, the rocker arm can be swung at a high speed.Additionally, there is no need to build a cam-switching mechanism intothe rocker arm, and therefore the structure of the rocker arm can bemade simple, and this makes it possible to heighten the rigidity of therocker arm. As a result, the rocker arm can accurately transmit theoperation of the cam to the intake valve or to the exhaust valve.

Additionally, the valve drive system is not arranged so as to move thevalve-driving cam of the camshaft in the axial direction. Therefore, thecamshaft can be produced without applying processing for moving thevalve-driving cam.

Additionally, even when the switching cam is formed integrally with thecamshaft, the camshaft can be more easily processed than the structureof Patent Document 3 that uses a spline to perform power transmissionand a movement in the axial direction. Therefore, production costs canbe made low.

In one embodiment of the present invention, the drive unit includes adriving mechanism that transforms rotation of the camshaft into a thrustforce toward one side or toward an opposite side in the axial directionof the camshaft, a slider that is driven by the driving mechanism tomove in the axial direction of the camshaft, a connecting mechanism thatconnects the slider and the rocker arm, and a holding mechanism thatholds the slider at a position to which the slider has moved.Preferably, in this case, the driving mechanism includes a first cammechanism that moves the slider toward one side in the axial directionwhen the amounts of valve lifts of the plurality of cams are 0, and asecond cam mechanism that moves the slider toward an opposite side inthe axial direction when the amounts of valve lifts of the plurality ofcams are 0. Preferably, the driving mechanism additionally includes anactuator that performs switching between “use” and “nonuse” of the firstand second cam mechanisms. Preferably, a movement distance of the slidermoved by the first and second cam mechanisms is set to be equal to theformation interval between the plurality of cams or is set to be a valueclose to the formation interval between the plurality of cams.

Additionally, in one embodiment of the present invention, each of thefirst cam mechanism and the second cam mechanism includes a switchingcam formed of a cam groove that has a predetermined depth in a radialdirection of the camshaft and that extends in a circumferentialdirection and in the axial direction of the camshaft, and a cam followerarranged so as to be guided by the switching cam. Preferably, in thiscase, the actuator is arranged so as to reciprocate the cam followers ofthe first and second cam mechanisms between a use position at which thecam followers are guided while being in contact with the switching camand a nonuse position at which the cam followers are apart from theswitching cam outwardly in the radial direction. Preferably, the slideris supported by a portion of the camshaft at which the switching cam isformed relatively rotatably with respect to the camshaft, and is heldsuch that rotation around the camshaft is restrained by the connectingmechanism. Preferably, the cam followers of the first and second cammechanisms are movably supported by the slider.

Preferably, the connecting mechanism is arranged so as to transmit athrust force from the slider to the rocker arm through the rocker shaft.

In one embodiment of the present invention, each of the switching camsof the first cam mechanism and the second cam mechanism includes amovement groove that has an inclined part used to move the slider in theaxial direction and an annular positioning groove that extends in thecircumferential direction of the camshaft at a same position in theaxial direction as a terminal of the inclined part. Preferably, in thiscase, the holding mechanism includes the positioning groove and the camfollower.

The positioning groove of the first cam mechanism and the positioninggroove of the second cam mechanism may be formed at a same position inthe axial direction. In other words, one positioning groove may beshared between the first cam mechanism and the second cam mechanism.

Preferably, a depth of the positioning groove is equal to or greaterthan a depth of the movement groove.

In one embodiment of the present invention, the actuator includes alifter for each cam follower that is attached to a front end of the camfollower and that is supported so as to enter and leave the slider, aspring member that presses the lifter in a direction in which the lifterleaves the slider, and an actuator body that faces the lifter.Preferably, in this case, the actuator body is supported by a cylinderhead or a head cover, and includes a plurality of plungers that proceedto and recede from the lifter.

In one embodiment of the present invention, the rocker shaft includes afirst rocker shaft that moves in the axial direction together with theslider and the rocker arm, and a second rocker shaft arranged so as tobe located coaxially with the first rocker shaft and so as to berelatively movable in the axial direction with respect to the firstrocker shaft. Preferably, the first rocker shaft is joined to the rockerarms corresponding to a plurality of cylinders of the engine so that thethrust force is transmitted thereto. Preferably, the first cam mechanismand the second cam mechanism are arranged so as to generate a thrustforce by which the slider is moved when the amounts of valve liftsbecome 0 in the plurality of cylinders.

In one embodiment of the present invention, the rocker shaft includes anouter rocker shaft that is shaped like a pipe and to which the rockerarm is attached, and an inner rocker shaft that is movably fitted to aninside of the outer rocker shaft. Preferably, in this case, theconnecting mechanism is arranged so as to transmit a thrust force fromthe slider to the rocker arm through the outer rocker shaft. Preferably,the holding mechanism includes a dent formed on an outer surface or onan inner surface of the outer rocker shaft, and an in-and-out memberthat is arranged so as to be able to go in and out of the dent and thatis arranged so as to be pressed against the dent by elasticity.

A power source of the actuator may be an electrically-operated drivingsource. A power source of the actuator may be a hydraulic drivingsource. Preferably, in either case, the actuator is arranged such that,in an OFF state, one of the first and second cam mechanisms reaches ause state, and a remaining one thereof reaches a nonuse state.

The aforementioned or other objects, features, and advantages of thepresent invention will become more apparent from the followingdescription of the embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing an arrangement of a valve drive system ofan engine according to an embodiment of the present invention.

FIG. 2 is an enlarged side view of a main part of the valve drivesystem, and shows a slider illustrated in a sectioned state (a hatchedstate).

FIG. 3 is a sectional view along line III-III of the main part of FIG.1.

FIG. 4 is a front view of a rocker arm part shown by arrow “A” of FIG.1.

FIG. 5 is a sectional view of a rocker shaft part at a position shown byline V-V of FIG. 3.

FIG. 6 is a perspective view of a camshaft.

FIGS. 7A, 7B, and 7C are graphs showing the crank angle, the amount ofvalve lift, and the position and depth of each groove of anordinarily-used in-line four-cylinder engine. FIG. 7A shows arelationship between the crank angle and the amount of valve lift ofeach cylinder, FIG. 7B shows a relationship among the crank angle, theamount of valve lift of each of first and second cylinders, the positionof each groove, and the depth of each groove, and FIG. 7C shows arelationship among the crank angle, the amount of valve lift of each ofthird and fourth cylinders, the position of each groove, and the depthof each groove.

FIGS. 8A, 8B, and 8C are side views for describing an operation forswitching between cams, and FIG. 8A shows a state before switchingtherebetween, FIG. 8B shows a state immediately after the switchingoperation of an actuator, and FIG. 8C shows a state in which a first camfollower has been inserted in a cam groove.

FIGS. 9A and 9B are side views for describing a cam switching operation,FIG. 9A shows a state in which a slider has started moving, and FIG. 9Bshows a state in which the slider has finished moving.

FIGS. 10A and 10B are side views showing an embodiment in which two offour cylinders are brought into a dormant state, FIG. 10A shows a statein which all cylinders are used and operated, and FIG. 10B shows a statein which two cylinders are dormant.

FIGS. 11A and 11B are side views showing an embodiment in which one oftwo valves that are provided for each cylinder is brought into a dormantstate, FIG. 11A shows a state in which all valves are used and operated,and FIG. 11B shows a state in which one of the two valves is dormant.

FIGS. 12A, 12B, and 12C are graphs showing a relationship between thecrank angle and the amount of valve lift of a V-type eight-cylinderengine, FIG. 12A shows a relationship therebetween concerning allcylinders, FIG. 12B shows a relationship therebetween concerningcylinders of a first bank, and FIG. 12C shows a relationshiptherebetween concerning cylinders of a second bank.

FIGS. 13A and 13B are side views showing an example in which onlycylinders placed at both ends of a group of four cylinders are broughtinto a dormant state, FIG. 13A shows a state in which all cylinders areused and operated, and FIG. 13B shows a state in which two cylinders aredormant.

FIG. 14 is a sectional view showing another embodiment of a holdingmechanism.

FIG. 15 is a sectional view showing still another embodiment of theholding mechanism.

FIG. 16 is a side view showing another embodiment of the cams, and showsthe slider illustrated in a sectioned state.

FIG. 17 is a side view showing another embodiment of the actuator, andshows a main part illustrated in a sectioned state.

FIG. 18 is a side view showing another embodiment of the slider, andshows apart of the slider illustrated in a sectioned state.

MODES FOR CARRYING OUT THE INVENTION First Embodiment

A first embodiment of a valve drive system of an engine according to thepresent invention will be hereinafter described in detail with referenceto FIG. 1 to FIG. 9B. The present embodiment is one example in which thepresent invention is applied to an in-line four-cylinder engine.

A valve drive system 1 of an engine shown in FIG. 1 is arranged so as todrive valves 2, two of which are provided for each cylinder, by means ofa camshaft 3 and a rocker arm 4. The valve 2 is an intake valve or anexhaust valve. The valve drive system 1 is applicable to an engine thatincludes an intake camshaft and an exhaust camshaft. The valve drivesystem 1 is applicable also to an engine that includes only onecamshaft. Therefore, in a description of the present embodiment, nodistinction is drawn between members of the intake system and members ofthe exhaust system.

The engine to which the valve drive system 1 is applied has two intakevalves or two exhaust valves for each cylinder. For convenience, in thedescription of the present embodiment, a cylinder leftmost in FIG. 1 isreferred to as a “first cylinder (#1 cylinder),” a cylinder rightwardlynext to the leftmost cylinder is referred to as a “second cylinder (#2cylinder),” a cylinder rightwardly next to the second cylinder isreferred to as a “third cylinder (#3 cylinder),” and a cylinderrightwardly next to the third cylinder is referred to as a “fourthcylinder (#4 cylinder).”

The camshaft 3 shown in FIG. 1 is supported rotatably around its axis bya cylinder head 5 and a cam cap 6.

An end of the camshaft 3 is connected to a crankshaft 10 of the enginethrough a transmission device 9. The camshaft 3 is contained in a valvedrive system chamber 8. The valve drive system chamber 8 is definedbetween the cylinder head 5 and a head cover 7 attached to the cylinderhead 5.

The camshaft 3 includes a plurality of cams, which differ in valve liftcharacteristics from each other, for each valve 2. These cams includelow-speed cams 11 each having a relatively small amount of valve liftand high-speed cams 12 each having a relatively great amount of valvelift. These cams 11 and 12 are arranged at a predetermined interval(pitch) in the axial direction of the camshaft 3. In other words, thesecams 11 and 12 are formed on the outer peripheral surface of thecamshaft 3 in a state of being adjacent to each other with apredetermined interval (pitch) therebetween.

In order to support sliders 15 of drive units 13 and 14 described later,the camshaft 3 is provided with two large diameter portions 16. Onelarge diameter portion 16 is disposed between the cams 11 and 12 for thefirst cylinder and the cams 11 and 12 for the second cylinder. The otherlarge diameter portion 16 is disposed between the cams 11 and 12 for thethird cylinder and the cams 11 and 12 for the fourth cylinder. As shownin FIG. 2 and FIG. 3, these two large diameter portions 16 are greaterin the outer diameter than a shaft portion 3 a of the camshaft 3. In thepresent embodiment, the large diameter portion 16 is formed integrallywith the shaft portion 3 a by integral molding as best shown in FIG. 3.However, the large diameter portion 16 may be a cylindrical body formedintegrally with the shaft portion 3 a by press fitting. In anarrangement formed by pressing and fitting the large diameter portion 16to the shaft portion 3 a, the cams 11 and 12 may be members arranged soas to be pressed and fitted to the shaft portion 3 a.

As shown in FIG. 2 to FIG. 5, the rocker arm 4 includes a rocker armbody 18, a presser 19, and a roller 20. The rocker arm body 18 isswingably supported by a rocker shaft 17 described later. The rocker armbody 18 is arranged so as to rock on the rocker arm shaft 17, andincludes a basal end 18 a joined to the rocker shaft 17 and a swing end18 b disposed apart from the rocker arm shaft 17. The presser 19 isdisposed integrally with the swing end 18 b of the rocker arm body 18.The roller 20 is rotatably attached to an intermediate portion 18 c ofthe rocker arm body 18.

As shown in FIG. 5, the rocker shaft 17 includes an outer rocker shaft21 shaped like a pipe and an inner rocker shaft 23 that is movablyfitted to the outer rocker shaft 21. The basal end 18 a of the rockerarm body 18 is rotatably joined to the outer rocker shaft 21 (see FIG.5) of the rocker shaft 17. Furthermore, the basal end 18 a of the rockerarm body 18 is sandwiched from both sides in the axial direction of therocker shaft 17 by a pair of E rings 22 attached to the outer rockershaft 21. In other words, the rocker arm body 18 is joined to the outerrocker shaft 21 so as not to be moved in the axial direction withrespect to the outer rocker shaft 21.

An oil passage 24 is defined in the axial center part of the innerrocker shaft 23. The oil passage 24 is arranged so that oil is suppliedfrom an oil supply passage (not shown) of the cylinder head 5. As shownin FIG. 1 and FIG. 5, in order to restrain the movement of the outerrocker shaft 21, E rings 25 are attached to both ends and anintermediate part of the inner rocker shaft 23, respectively.

In the present embodiment, the rocker shaft 17 includes one inner rockershaft 23 and two outer rocker shafts 21 and 21.

As shown in FIG. 1, four rocker arms 4 corresponding to the first andsecond cylinders #1 and #2 are swingably joined to one of the two outerrocker shafts 21 and 21. On the other hand, four rocker arms 4corresponding to the third and fourth cylinders #3 and #4 are swingablyjoined to the other outer rocker shaft 21. These two outer rocker shafts21 are relatively movable in the axial direction with respect to theinner rocker shaft 23 in a range defined by the E rings 25. Therefore,the outer rocker shaft 21 is supported movably with respect to thecylinder head 5 through the inner rocker shaft 23. In other words, therocker arm 4 joined to the outer rocker arm 21 is movable in the axialdirection of the rocker shaft 17 with respect to the cylinder head 5.The rocker arm 4 is arranged so as to be moved in the axial direction bythe drive units 13 and 14 (described later) that are disposed at partsdiffering from the rocker arm 4.

The presser 19 of the rocker arm 4 is arranged so as to press a forwardend of the valve 2. A cap-like shim 26 and a retainer 27 are attached tothe forward end of the valve 2. The valve 2 is pressed in a closingdirection (upwardly in FIG. 1) by a valve spring 28 (see FIG. 2)interposed between the retainer 27 and the cylinder head 5.

As shown in FIG. 2, the presser 19 is formed in a shape that extends inthe axial direction of the rocker shaft 17. The length in the axialdirection of the presser 19 is greater than a formation interval (pitch)between the low-speed cam 11 and the high-speed cam 12. The formationinterval is a distance between the center in the cam-width direction(axial direction) of the cam 11 and the center in the cam-widthdirection of the cam 12, i.e., is a formation pitch between the two cams11 and 12.

The roller 20 of the rocker arm 4 is arranged so as to rotate whilebeing in contact with either of the low-speed cam 11 and the high-speedcam 12. The roller 20 is pressed by the low-speed cam 11 or thehigh-speed cam 12, and, as a result, the rocker arm 4 rocks on therocker shaft 17, and depresses the valve 2. In the present embodiment,the width in the axial direction of the roller 20 is equal to or smallerthan the width of the low-speed cam 11 or the width of the high-speedcam 12.

The drive units 13 and 14 are arranged so as to move the rocker arm 4 inthe axial direction of the rocker shaft 17 so that either of thelow-speed cam 11 and the high-speed cam 12 is used. More specifically,the drive units 13 and 14 are arranged so as to move the rocker arm 4 bymoving the outer rocker shaft 21 in the axial direction. The drive units13 and 14 are arranged so as to move the outer rocker shaft 21 in theaxial direction when the amount of valve lift is 0 in both the low-speedcam 11 and the high-speed cam 12.

In the valve drive system 1 according to the present embodiment, theamount of valve lift of each cylinder varies as shown in FIG. 7A.

As is understood from FIG. 7A, the amount of valve lift of the secondcylinder #2 becomes 0 for a relatively long period when the amount ofvalve lift is 0 in the first cylinder #1. The amount of valve lift ofthe fourth cylinder #4 becomes 0 for a relatively long period when theamount of valve lift is 0 in the third cylinder #3. Therefore, the valvedrive system 1 according to the present embodiment forms a first groupby the first and second cylinders #1 and #2, and forms a second group bythe third and fourth cylinders #3 and #4. The rocker arms 4 of the firstgroup are arranged so as to be driven by the drive unit 13, whereas therocker arms 4 of the second group are arranged so as to be driven by theother drive unit 14.

In more detail, the rocker arms 4 corresponding to the first and secondcylinders #1 and #2 are arranged so as to be moved in the axialdirection of the rocker shaft 17 by the drive unit 13 as shown inFIG. 1. The drive unit 13 is disposed between the cams 11 and 12 for thefirst cylinder and the cams 11 and 12 for the second cylinder. Therocker arms 4 corresponding to the third and fourth cylinders #3 and #4are arranged so as to be moved in the axial direction of the rockershaft 17 by the drive unit 14. The drive unit 14 is disposed between thecams 11 and 12 for the third cylinder and the cams 11 and 12 for thefourth cylinder.

The drive units 13 and 14 are substantially the same in arrangementalthough these are different in operation timing. Therefore, adescription is here given of the drive unit 13 that moves the rocker arm4 for the first cylinder and the rocker arm 4 for the second cylinder.The same reference numeral as in the drive unit 13 is given to eachcorresponding part of the other drive unit 14, and a detaileddescription of the drive unit 14 is omitted.

As shown in FIG. 2 and FIG. 6, a switching cam 31 having cam grooves isformed on the large diameter portion 16 of the camshaft 3. The driveunit 13 is arranged so as to generate a thrust force in the axialdirection of the camshaft 3 by using the switching cam 31 and so as tomove the rocker arm 4 in the axial direction of the rocker shaft 17 bythe thrust force. In other words, the drive unit 13 is arranged so as tomove the rocker arm 14 toward one side or toward an opposite side in theaxial direction of the rocker shaft 17 over a distance equivalent to theformation interval between the low-speed cam 11 and the high-speed cam12. Arrows in FIG. 2 and FIG. 6 represent the rotation direction of thecamshaft 3.

In the present embodiment, the drive unit 13 includes the slider 15, adriving mechanism 34, the outer rocker shaft 21, and a holding mechanism35 as shown in FIG. 2 and FIG. 3. The slider 15 is movably supported bythe large diameter portion 16 of the camshaft 3. The driving mechanism34 has first and second cam mechanisms 32 and 33 that generate theabove-mentioned thrust force. The outer rocker shaft 21 forms aconnecting mechanism that connects the slider 15 and the rocker arm 4together. The holding mechanism 35 is arranged so as to hold the slider15 at a position to which the slider 15 has moved. Arrows in FIG. 2 andFIG. 3 represent the rotation direction of the camshaft 3.

As shown in FIG. 3, the slider 15 includes an upper half portion 41 anda lower half portion 43 attached to the upper half portion 41 with bolts42. The upper half portion 41 and the lower half portion 43 arerotatably supported by the outer peripheral surface of the largediameter portion 16 in a state of sandwiching the large diameter portion16 of the camshaft 3 from one side and from an opposite side in a radialdirection (i.e., in an up-down direction in FIG. 3). In actual practice,the slider 15 is kept in a non-rotational state when the camshaft 13rotates, whereas the large diameter portion 16 rotates around its axisbetween the upper half portion 41 and the lower half portion 43.

A cam follower supporting portion 41 a that protrudes outwardly in theradial direction of the camshaft 3 is formed on the upper half portion41. As shown in FIG. 2, the cam follower supporting portion 41 asupports first and second cam followers 44 and 45, lifters 47 and 48,and a spring member 49. The first and second cam followers 44 and 45 arecylindrical members, respectively, and serve as parts of the first andsecond cam mechanisms 32 and 33, respectively. The lifters 47 and 48serve as a part of an actuator 46 (described later) that drives the camfollowers 44 and 45. The spring member 49 is arranged so as to press thelifters 47 and 48 in a direction going out from the slider 15 (i.e.,outwardly in the radial direction of the camshaft 3).

As shown in FIG. 3, an arm 51 used to connect the slider 15 to therocker shaft 17 is formed on the lower half portion 43. A forward end 51a of the arm 51 is shaped like the capital letter C in the cross sectionthat is opened toward the rocker shaft 17. The forward end 51 a isfitted into an annular groove 52 of the outer rocker shaft 21. Theannular groove 52 is a groove that extends in the circumferentialdirection of the outer rocker shaft 21. The forward end 51 a of the arm51 is joined to the annular groove 52, and, as a result, the slider 15is prevented from being rotated together with the camshaft 3. In otherwords, the slider 15 is kept in a non-rotational state when the camshaft3 rotates.

The forward end 51 a of the arm 51 is fitted into the annular groove 52so as not to be moved in the axial direction of the outer rocker shaft21. Therefore, the slider 15 and the outer rocker shaft 21 move togetherwith each other in the axial direction of the camshaft 3. In the presentembodiment, the outer rocker shaft 21 serves as a first rocker shaftthat moves together with the slider 15 and the rocker arm 4 in the axialdirection. On the other hand, the inner rocker shaft 23 serves as asecond rocker shaft that is arranged so as to be located coaxially withthe first rocker shaft and so as to be relatively movable in the axialdirection with respect to the first rocker shaft.

As shown in FIG. 3, an oil passage 53 is defined in the arm 51. An endof the oil passage 53 is opened toward an inner peripheral surface ofthe lower half portion 43. The inner peripheral surface of the lowerhalf portion 43 faces the large diameter portion 16. The other end ofthe oil passage 53 is connected to the oil passage 24 inside the innerrocker shaft 23 through an oil hole 54 of the outer rocker shaft 21 andthrough an oil hole 55 of the inner rocker shaft 23. In other words, oilsupplied to the oil passage 24 inside the inner rocker shaft 23 isguided through the oil holes 54 and 55 and through the oil passage 53 toa place between the slider 15 and the large diameter portion 16, andthis place is lubricated with the oil.

As shown in FIG. 2, the driving mechanism 34 includes the first cammechanism 32, the second cam mechanism 33, and the actuator 46. Thefirst cam mechanism 32 is arranged so as to move the slider 15 towardone side (i.e., rightwardly in FIG. 2) in the axial direction of therocker arm 17. The second cam mechanism 33 is arranged so as to move theslider 15 toward an opposite side in the axial direction. The actuator46 is arranged so as to perform switching between use and nonuse of thefirst and second cam mechanisms 32 and 33.

The first cam mechanism 32 includes the switching cam 31 formed in agroove shape on the large diameter portion 16 and the first cam follower44 that is engaged with the switching cam 31. Likewise, the second cammechanism 33 includes the switching cam 31 formed in a groove shape onthe large diameter portion 16 and the second cam follower 45 that isengaged with the switching cam 31.

The switching cam 31 includes a cam groove that extends in thecircumferential direction and in the axial direction of the camshaft 3and that has a depth in the radial direction of the camshaft 3. In moredetail, as shown in FIG. 2 and FIG. 6, the switching cam 31 includes apair of movement grooves 57 and a positioning groove 58. The movementgroove 57 has an inclined part 56 used to move the slider 15 in theaxial direction of the camshaft 3. The positioning groove 58 extendsover the whole circumference of the camshaft 3 at the same position inthe axial direction as the terminal (the lower end in FIG. 2) of themovement groove 57.

In the present embodiment, the positioning groove 58 of the first cammechanism 32 and the positioning groove 58 of the second cam mechanism33 are formed at the same position in the axial direction of thecamshaft 3. In other words, in the driving mechanism 34 according to thepresent embodiment, one positioning groove 58 is shared between thefirst cam mechanism 32 and the second cam mechanism 33. However, thepositioning groove 58 of the first cam mechanism 32 and the positioninggroove 58 of the second cam mechanism 33 may be cam grooves differingfrom each other that are spaced out in the axial direction of thecamshaft 3.

In the present embodiment, the holding mechanism 35 is arranged by thepositioning groove 58 and the first and second cam followers 44 and 45.

As shown in FIG. 2 and FIG. 6, each of the two movement grooves 57 has alinear part 59 that extends in the circumferential direction of thecamshaft 3 and the inclined part 56 that is inclined with respect to thecircumferential direction. Each movement groove 57 is formed in anon-annular shape in which one end of the linear part 59 is used as astart end and in which one end of the inclined part 56 is used as aterminal. The inclined part 56 is inclined so as to be graduallydisplaced in the axial direction of the camshaft 3 correspondingly toprogress in the circumferential direction. The inclined part 56 of thefirst cam mechanism 32 and the inclined part 56 of the second cammechanism 33 are inclined in mutually opposite directions.

The first and second cam followers 44 and 45 are arranged so as to beengaged with the switching cam 31. The first and second cam followers 44and 45 are supported by the cam follower supporting portion 41 a of theslider 15 so as to be movable in the radial direction of the camshaft 3.The first and second cam followers 44 and 45 are arranged so as to bemoved by the actuator 46 inside the slider 15.

The first and second cam followers 44 and 45 are arranged so as toreciprocate between a use position and a nonuse position by being drivenby the actuator 46. The use position is a position at which the camfollowers 44 and 45 are fitted to the switching cam 31 formed of the camgrooves. The nonuse position is a position at which the cam followers 44and 45 are apart from the switching cam 31 outwardly in the radialdirection of the camshaft 3.

When the camshaft 3 rotates in a state in which one of the first andsecond cam followers 44 and 45 is located at the use position and hasentered the movement groove 57, the one cam follower is guided by theinclined part 56 of the switching cam 31. As a result, the slider 15moves toward one side or toward an opposite side in the axial directionof the camshaft 3. An interval between the positioning groove 58 and thelinear part 59 of the movement groove 57 is set so that the movementdistance of the slider 15 reaches a formation interval between thelow-speed cam 11 and the high-speed cam 12 or reaches a value close tothis formation interval.

The movement groove 57 (especially, the inclined part 56) of the firstcam mechanism 32 and the movement groove 57 (especially, the inclinedpart 56) of the second cam mechanism 33 are formed at the same positionwith respect to the circumferential direction of the camshaft 3 (thelarge diameter portion 16). As shown in FIG. 7B, the position in thecircumferential direction of the camshaft 3 at which the movementgrooves 57 (especially, inclined parts 56) are formed is a position atwhich the amount of valve lift of the cam for the first cylinder #1 andthe amount of valve lift of the cam for the second cylinder #2 bothbecome 0. In other words, the first and second cam followers 44 and 45pass along the movement groove 57 (especially, inclined part 56) in acommon 0-lift section of the first and second cylinders #1 and #2 shownin FIG. 7B.

Therefore, the slider 15 is arranged so as to move in the axialdirection of the camshaft 3 when the roller 20 of the rocker arm 4 facesa basic circle part of the low-speed/high-speed cams 11 and 12 (i.e., aplace at which the amount of valve lift becomes 0). In the presentembodiment, as shown in FIG. 7B, a period (section margin) is providedduring which the amount of valve lift becomes 0 by a predetermined crankangle before and after a period (movement section) during which theslider 15 moves.

As shown in the depth of the groove of FIG. 7B, the movement groove 57is formed so as to gradually become deeper in proportion to progress inthe direction in which the camshaft 3 rotates and so as to finally havethe same depth as the positioning groove 58. The depth of thepositioning groove 58 is formed so as to be constant over the wholecircumference. However, it is possible to form the depth of thepositioning groove 58 greater than the depth of the movement groove 57.In other words, it is possible to form the depth so that 0<h1(h2)≦hwhere h1 and h2 are the depths of the terminals of both movement grooves57, and h is the depth of the positioning groove 58.

On the other hand, the drive unit 14 for the third and fourth cylinders#3 and #4 is arranged so that the slider 15 moves in a common 0-liftsection between the third cylinder #3 and the fourth cylinder #4 asshown in FIG. 7C.

The first and second cam followers 44 and 45 are arranged so as to bedriven by the actuator 46. As shown in FIG. 2, the actuator 46 includesthe first and second lifters 47 and 48, the spring member 49, and anactuator body 60. The first and second lifters 47 and 48 are attached tothe front ends of the first and second cam followers 44 and 45,respectively. A pair of spring members 49 are provided correspondinglyto the lifters 47 and 48, and press the lifters 47 and 48 against theactuator body 60. The actuator body 60 is disposed so as to face thelifters 47 and 48.

The first and second lifters 47 and 48 are shaped to be cylindrical, andare movably fitted to a pair of circular holes 41 b formed in the slider15, respectively. The forward ends of the lifters 47 and 48 protrudeoutwardly from the slider 15. In the present embodiment, the springmembers 49 are compression coil springs, and are disposed between thelifter 47 and the slider 15 and between the lifter 48 and the slider 15(the bottom surface of the circular hole 41 b), respectively.

The actuator body 60 is composed of first and second plungers 60 a and60 b that are cylindrical and that face the lifters 47 and 48,respectively, and a solenoid 60 c that drives the plungers 60 a and 60b. The actuator body 60 is supported by the cylinder head 5 or the headcover 7.

The first and second plungers 60 a and 60 b are allowed to proceed to orrecede from the corresponding lifters 47 and 48 by being driven by thesolenoid 60 c. The lifters 47 and 48 are pressed by the correspondingspring members 49 and, as a result, are brought into contact with theplungers 60 a and 60 b, respectively.

The solenoid 60 c is arranged so as to allow one of the plungers 60 aand 60 b to proceed and allow the other one to recede in an OFF statethat is a non-excitation state. In other words, the actuator 46 isarranged so that one of the first and second cam mechanisms 32 and 33reaches a use state, whereas the other one reaches a nonuse state in anOFF state.

In the actuator 46, when the slider 15 moves toward one side or towardan opposite side in the axial direction of the camshaft 3, the lifters47 and 48 move while being in contact with the plungers 60 a and 60 b.The outer diameters and the installation intervals (pitches) of both ofthe lifters 47 and 48 and the outer diameters and the installationintervals (pitches) of both of the plungers 60 a and 60 b are set sothat the lifters 47 and 48 never come off from the plungers 60 a and 60b when the slider 15 moves. Additionally, the outer diameters and theinstallation intervals (pitches) of these components are set so that thefirst lifter 47 comes into contact with only the first plunger 60 a andso that the second lifter 48 comes into contact with only the secondplunger 60 b.

The operation of the valve drive system 1 arranged in this way will bedescribed with reference to FIGS. 8A to 8C and FIGS. 9A and 9B. Herein,a description is given of the operation thereof when switching isperformed from a state of using the low-speed cam 11 to a state of usingthe high-speed cam 12.

As shown in FIG. 8A, when the low-speed cam 11 is used, the secondplunger 60 b of the actuator 46 moves forwardly, and the second camfollower 45 is inserted in the positioning groove 58. In order to allowthe high-speed cam 12 to be used from this state, the second plunger 60b is first of all retreated by the actuator 46. As a result of theretreat of the second plunger 60 b, the second cam follower 45 is movedto the nonuse position by the force of the spring member 49.

Thereafter, the first plunger 60 a is advanced by the actuator 46. As aresult of the advancement of the first plunger 60 a, the first camfollower 44 is pressed toward the use position. At this time, there area case in which the first cam follower 44 directly enters the movementgroove 57 of the first cam mechanism 32 and a case in which the firstcam follower 44 presses an outer peripheral surface of the largediameter portion 16 that is located upstream of the movement groove 57in the rotation direction. In the latter case, the first cam follower 44goes into the linear part 59 of the movement groove 57 from the startend by the rotation of the camshaft 3 (see FIG. 8B).

As shown in FIG. 8C and FIG. 9A, the rotation of the camshaft 3 enablesthe first cam follower 44 to advance from the linear part 59 of the cam31 to the inclined part 56 in the movement groove 57. The first camfollower 44 is pressed by the cam 31 toward one side (rightwardly inFIG. 9A) in the axial direction of the camshaft 3 when passing along theinclined part 56. In response to the pressing of the first cam follower44 in this way, the slider 15 moves in the direction in which the firstcam follower 44 is pressed. As a result, the outer rocker shaft 21 andthe rocker arm 4 move in the same direction together with the slider 15.

Thereafter, the first cam follower 44 enters the inside of thepositioning groove 58 from the inclined part 56 as shown in FIG. 9B. Asa result of the entrance of the first cam follower 44 into thepositioning groove 58, the high-speed cam 12 presses the roller 20 ofthe rocker arm 4, and the switching operation between the cams 11 and 12is completed. Additionally, it becomes impossible for the slider 15 tomove in the axial direction of the camshaft 3.

On the other hand, when switching of the cams to be used from thehigh-speed cam 12 to the low-speed cam 11 is performed, the firstplunger 60 a of the actuator 46 is retreated, and the second plunger 60b is advanced. As a result, the same operation as above is performed. Indetail, the first cam follower 44 is retreated to the nonuse position,whereas the second cam follower 45 is pressed toward the use position,and enters the inside of the movement groove 57 of the second cammechanism 33. The second cam follower 45 advances from the linear part59 of the cam 31 to the inclined part 56 in the movement groove 57 bythe rotation of the camshaft 3. The second cam follower 45 is pressedtoward one side in the axial direction of the camshaft 3 (leftwardly inFIG. 8A) by the cam 31 when passing along the inclined part 56. Inresponse to the pressing of the second cam follower 45 in this way, theslider 15 moves in the direction in which the second cam follower 45 ispressed. As a result, the outer rocker shaft 21 and the rocker arm 4move in the same direction together with the slider 15. Thereafter, thesecond cam follower 45 enters the inside of the positioning groove 58from the inclined part 56. As a result of the entrance of the second camfollower 45 into the positioning groove 58, the low-speed cam 11 pressesthe roller 20 of the rocker arm 4, and the switching operation betweenthe cams 11 and 12 is completed. Additionally, it becomes impossible forthe slider 15 to move in the axial direction of the camshaft 3.

In the present embodiment, the rocker arm 4 is moved in the axialdirection of the rocker shaft 17 by being driven by the drive units 13and 14 that are disposed at parts differing from the rocker arm 4, andfaces either of the low-speed cam 11 and the high-speed cam 12. Asdescribed above, the moving components that perform switching betweenthe cams to be used are disposed at parts differing from the rocker arm4, and therefore it is possible to restrain an increase in mass of therocker arm 4.

Therefore, it becomes possible for the rocker arm 4 to rock at a highspeed. Additionally, the rocker arm 4 does not have a switchingmechanism that performs switching between cams to be used, and thereforedesigning can be easily performed to form a structure having a highrigidity. Therefore, the rocker arm 4 can accurately transmit theoperation of the low-speed cam 11 or the high-speed cam 12 to the valve2.

The valve drive system 1 according to the present embodiment is notarranged so as to move the valve-driving cams of the camshaft 3 in theaxial direction. Therefore, the camshaft 3 can be produced withoutapplying processing for moving the low-speed cam 11 or the high-speedcam 12. Additionally, the movement groove 57 (the switching cam 31) ofthe camshaft 3 can be formed more easily than a spline used to performpower transmission and movement in the axial direction.

Therefore, the valve drive system 1 according to the present embodimentuses the camshaft 3 that is easily produced, and hence can be producedat a relatively low cost.

Additionally, in the present embodiment, the drive unit 13 is composedof the driving mechanism 34, the slider 15, and the connecting mechanism(the outer rocker arm 21). The driving mechanism 34 is composed of thefirst cam mechanism 32, the second cam mechanism 33, and the actuator46. The movement distance of the slider 15 moved by the first and secondcam mechanisms 32 and 33 is set to be equal to or be close to theformation interval (pitch) between the cams 11 and 12.

In the drive units 13 and 14 arranged in this way, switching between themovement and the stopping of the rocker arm 4 in the axial direction isperformed by the first and second cam mechanisms 32 and 33.

In other words, in the valve drive system 1 according to the presentembodiment, a rigid body never collides with another rigid body bymoving in the axial direction of the camshaft 3 when switching betweenthe valve-driving cams 11 and 12 is performed. Therefore, the rocker arm4 smoothly moves in the axial direction of the camshaft 3. Whenswitching between the low-speed cam 11 and the high-speed cam 12 isperformed, an impact sound never occurs, or is remarkably low even ifsuch an impact sound occurs.

In the present embodiment, the first cam mechanism 32 and the second cammechanism 33 of the driving mechanism 34 include the switching cams 31and the first and second cam followers 44 and 45, respectively. Theactuator 46 is arranged so as to reciprocate the first and second camfollowers 44 and 45 between the use position and the nonuse position.The slider 15 is rotatably supported by the large diameter portion 16 ofthe camshaft 3, and its rotation is restrained by the rocker shaft 17.The first and second cam followers 44 and 45 are movably supported bythe slider 15.

Therefore, the slider 15 moves in the axial direction of the rockershaft 17 while being supported by the camshaft 3 and the rocker shaft17. In other words, the direction in which the slider 15 moves isrestrained by the camshaft 3 and the rocker shaft 17 that are theexisting members.

Therefore, in the valve drive system 1 according to the presentembodiment, the number of components becomes smaller than in anarrangement in which a dedicated guide member is used to restrain thedirection in which the slider 15 moves, and production costs can be madesmaller.

In the present embodiment, a connecting mechanism arranged so as totransmit a thrust force from the slider 15 to the rocker arm 4 throughthe outer rocker shaft 21 is provided. As shown in FIG. 1, the outerrocker shaft 21 can support the rocker arms 4 corresponding to aplurality of cylinders together. In other words, the thrust force of thedrive unit 13 is transmitted to the rocker arms 4 corresponding to aplurality of cylinders through the outer rocker shaft 21.

Therefore, in the valve drive system 1 according to the presentembodiment, switching between the cams 11 and 12 for a plurality ofcylinders can be performed by the single drive unit 13. As a result, ifthe valve drive system 1 is applied to a multi-cylinder engine,production costs can be made lower than a valve drive system required tohave a drive unit for each cylinder.

Additionally, in the present embodiment, each switching cam 31 of thefirst and second cam mechanisms 32 and 33 includes the movement groove57 and the positioning groove 58. The first and second cam followers 44and 45 that are engaged with these grooves are guided to the positioninggroove 58 after passing along the movement groove 57. The movement ofthe slider 15 in the axial direction of the camshaft 3 is restrained bythe positioning groove 58 and the first and second cam followers 44 and45 inserted in the positioning groove 58.

Therefore, a load in the axial direction is not needed to restrain themovement in the axial direction of the slider 15, the outer rocker shaft21, and the rocker arm 4 after performing switching between the cams 11and 12. As a result, a slide loss can be prevented, and therefore thepower loss of the engine can be made small. Additionally, the number ofcomponents can be reduced, and the valve drive system can be made smallin size by sharing the mechanisms used for the movement and thepositioning of the slider 15.

Additionally, in the present embodiment, the positioning groove 58 ofthe first cam mechanism 32 and the positioning groove 58 of the secondcam mechanism 33 are formed at the same position with respect to theaxial direction of the camshaft 3. Therefore, the drive unit 13 is madesmall in the axial direction of the camshaft 3. The reason is that thefirst cam mechanism 32 and the second cam mechanism 33 are disposedclose to each other in the axial direction of the camshaft 3 by makingthe positioning grooves 58 common thereto.

In the present embodiment, the depth of the positioning groove 58 isequal to or is greater than the depth of the movement groove 57. If thepositioning groove 58 is deeper than the movement groove 57, the frontends of the first and second cam followers 44 and 45 can be preventedfrom pressing the groove bottom of the positioning groove 58 toward theaxial center of the camshaft 3. Therefore, in this case, the power lossof the engine becomes even smaller.

In the present embodiment, the actuator 46 includes the lifters 47 and48 for each cam follower, the spring member 49, and the actuator body60. The actuator body 60 is supported by the cylinder head 5 or the headcover 7, and is composed of the plurality of plungers 60 a and 60 b thatadvance to and retreat from the lifters 47 and 48, respectively.

According to the present embodiment, the weight of the actuator body 60that is a power source of the actuator 46 never acts on the slider 15,and is supported by the cylinder head 5 or the head cover 7.

Therefore, an inertia force that occurs when the slider 15 moves in theaxial direction of the camshaft 3 becomes smaller than in a case inwhich the actuator body 60 is supported by the slider 15. Therefore,according to the present embodiment, an impact sound does not occur evenif the slider 15 moves at a high speed.

The actuator body 60 is fixed to the cylinder head 5 or to the headcover 7 so as not to be moved. Therefore, according to the presentembodiment, the actuator 46 is stably supported, and therefore a highreliability is achieved when the actuator 46 operates.

The power source of the actuator 46 according to the present embodimentis an electrically-operated driving source that uses the solenoid 60 c.Therefore, a hydraulic passage is not needed, and the capacity of an oilpump may be smaller than in a case in which oil pressure is used as thepower source of the actuator 46. This makes it possible to achieve areduction in cost and in weight.

Additionally, in the present embodiment, the actuator 46 is arrangedsuch that, in an OFF state, one of the first and second cam mechanisms32 and 33 reaches a use state, and the other one reaches a nonuse state.Therefore, when the power of the actuator 46 is lost, the slider 15 iskept in a state of having moved toward one side in the axial directionof the crankshaft 3. Therefore, in the valve drive system 1 according tothe present embodiment, the slider 15 does not move needlessly, andswitching between the cams 11 and 12 is not performed needlessly even ifthe power of the actuator 46 is lost.

Additionally, in the present embodiment, the rocker shaft 17 has a dualstructure including the outer rocker shaft 21 and the inner rocker shaft23. Therefore, the rocker shaft 17 has a high rigidity. As a result, theoperation of the valve-driving cams (the low-speed cam 11 and thehigh-speed cam 12) is transmitted to the intake valve or to the exhaustvalve through the rocker arm 4 even more accurately.

Second Embodiment

The valve drive system of the present invention can be used to performswitching an active cam for an operating state and a dormant cam for adormant state. An embodiment employing this arrangement will bedescribed in detail with reference to FIGS. 10A and 10B and FIGS. 11Aand 11B. In these figures, the same reference numeral is given to thesame or equivalent member as in FIG. 1 to FIG. 9B, and a detaileddescription of the same or equivalent member is omitted.

The valve drive system 61 shown in FIGS. 10A and 10B is composed of adrive unit 13 that performs switching between cams 62 for the secondcylinder #2 and a drive unit 14 that performs switching between cams 62for the third cylinder #3. The cams 62 for the second and thirdcylinders #2 and #3 include active cams 63 and dormant cams 64,respectively.

With respect to the first cylinder #1 and the fourth cylinder #4, onlythe active cams 65 are provided, and the dormant cams are not provided.The active cams 63 provided correspondingly to the second and thirdcylinders #2 and #3 are each arranged so as to have the same shape thatdiffers in phase from that of each cam 65 for the first and fourthcylinders #1 and #4.

The dormant cam 64 is formed in a disk shape that has the same diameteras the basic circle portion of the active cam 63 in which the amount ofvalve lift becomes 0. In other words, the dormant cam 64 is arranged sothat the amount of valve lift becomes 0 without depending on therotation angle (phase) of the crankshaft 3.

The valve drive system 61 according to the present embodiment isarranged so as to perform switching between the active cam 63 and thedormant cam 64 in the cams to be used. An engine having this valve drivesystem 61 is a four-cylinder engine when the active cam 63 is used (seeFIG. 10A).

On the other hand, when the rocker arm 4 is moved to a positioncorresponding to the dormant cam 64 by being driven by the drive units13 and 14, the valve 2 is kept in a closed state in the second cylinder#2 and the third cylinder #3. Therefore, the second cylinder #2 and thethird cylinder #3 reach a dormant state (see FIG. 10B). In other words,in this state, fuel efficiency can be improved because the four-cylinderengine substantially becomes a two-cylinder engine.

The valve drive system 71 of FIG. 11 is arranged so as to performswitching between “active” and “dormant” concerning one of two valves 2per cylinder. The drive unit 13 of the two drive units 13 and 14 that islocated between the cams 72 for the first cylinder #1 and the cams 73for the second cylinder #2 is arranged so as to perform switchingbetween “active” and “dormant” of the two valves 2. One valve for whichswitching is performed is a valve 2A of the two valves 2 of the firstcylinder #1 that is closer to the second cylinder #2. Another valve forwhich switching is performed is a valve 2B of the two valves 2 of thesecond cylinder #2 that is closer to the first cylinder #1.

The drive unit 14 that is located between the cams 74 for the thirdcylinder #3 and the cams 75 for the fourth cylinder #4 is arranged so asto perform switching between “active” and “dormant” of another twovalves. One valve for which switching is performed is a valve 2C of thetwo valves 2 of the third cylinder #3 that is closer to the fourthcylinder #4. Another valve for which switching is performed is a valve2D of the two valves 2 of the fourth cylinder #4 that is closer to thethird cylinder #3.

The valve 2 for which switching between “active” and “dormant” isperformed is hereinafter referred to as the “switching valve 2A, 2B, 2C,or 2D.”

The cams 72 to 75 corresponding to the switching valves 2A to 2D includeactive cams 72 a to 75 a and dormant cams 72 b to 75 b, respectively.

The active cams 72 a to 75 a are each formed in the same shape thatdiffers in phase from the cams 72 to 75 that drive the valve 2 that is avalve for which switching between “active” and “dormant” is notperformed.

The dormant cams 72 b to 75 b are each formed in a disk shape that hasthe same diameter as the basic circle portion of each of the active cams72 a to 75 a in which the amount of valve lift becomes 0. In otherwords, the dormant cams 72 b to 75 b are each formed so that the amountof valve lift becomes 0.

The rocker arm 4 corresponding to each of the switching valves 2A to 2Dis connected to the slider 15 of the drive unit 13 through the outerrocker shaft 21. The rocker arm 4 corresponding to the valve 2 that is avalve for which switching between “active” and “dormant” is notperformed is supported by a fixed-type outer rocker shaft 21 a formed tobe structurally independent of the outer rocker shaft 21. Thisfixed-type outer rocker shaft 21 a is held by the cylinder head 5 andthe inner rocker shaft 23 so as not to be moved in the axial direction.

In the engine having the valve drive system 71 of FIG. 11, switching canbe performed between a normal operation mode in which two valves 2 areopened and closed in each cylinder and a unilateral valve dormant modein which only one valve 2 is opened and closed in each cylinder.

When the intake valve is driven while using the valve drive system 71, aswirl can be generated in a combustion chamber (not shown) by choosingthe unilateral valve dormant mode. The reason is that intake air isinhaled only from one of two intake ports in each cylinder, and the flowvelocity of intake air flowing through the intake port rises.

Third Embodiment

The valve drive system of the present invention can be used in a V-typeeight-cylinder engine. An embodiment of the valve drive systemapplicable to the V-type eight-cylinder engine will be described indetail with reference to FIGS. 12A to 12C and FIGS. 13A and 13B. InFIGS. 13A and 13B, the same reference numeral is given to the same orequivalent member as in FIG. 1 to FIG. 9B, and a detailed description ofthe same or equivalent member is omitted.

The valve drive system 81 (see FIGS. 13A and 13B) according to thepresent embodiment is arranged so as to perform switching betweenoperation modes of the V-type eight-cylinder engine. One operation modeis a mode in which the V-type eight-cylinder engine is used as a V-typeeight-cylinder engine. Another operation mode is a mode in which theV-type eight-cylinder engine is used substantially as a V-typefour-cylinder engine by decreasing the number of driven cylinders. TheV-type eight-cylinder engine has two cylinder rows each of whichconsists of four cylinders, and these two cylinder rows are arranged ina V shape. FIGS. 13A and 13B show a valve drive system used for onecylinder row of the V-type eight-cylinder engine.

The V-type eight-cylinder engine has first to eighth cylinders arrangedalong a direction from one end to an opposite end of a crankshaft. Ingeneral, one cylinder row (hereinafter, this cylinder row is referred toas “bank 1”) of the V-type eight-cylinder engine consists of a firstcylinder #1, a third cylinder #3, a fifth cylinder #5, and a seventhcylinder #7 as shown in FIG. 12A and FIG. 12B. The other bank (bank 2)consists of a second cylinder #2, a fourth cylinder #4, a sixth cylinder#6, and an eighth cylinder #8 as shown in FIG. 12C.

This V-type eight-cylinder engine is ignited generally in the followingorder.

First cylinder #1→Eighth cylinder #8→Seventh cylinder #7→Third cylinder#3→Sixth cylinder #6→Fifth cylinder #5→Fourth cylinder #4→Secondcylinder #2.

In order to bring cylinders into a dormant state in this V-typeeight-cylinder engine, it is preferable to choose a dormant cylinder soas to bring expansion strokes into equal intervals. The reason is thatthe occurrence of vibrations resulting from the worsening of rotationbalance is prevented. In order to bring expansion strokes into equalintervals, it is necessary to make the cylinders dormant alternately inthe ignition order.

A first cylinder group that has an alternate ignition order includes thefirst cylinder #1, the fourth cylinder #4, the sixth cylinder #6, andthe seventh cylinder #7. A second cylinder group that has an alternateignition order includes the second cylinder #2, the third cylinder #3,the fifth cylinder #5, and the eighth cylinder #8. In order to performswitching between the operation modes, switching between “operating” and“halt” must be performed in cylinders that belong to one of the firstand second cylinder groups.

Cylinders that belong to the first cylinder group in bank 1 are thefirst cylinder #1 and the seventh cylinder #7 as shown in FIG. 12B.Cylinders that belong to the first cylinder group in bank 2 are thefourth cylinder #4 and the sixth cylinder #6 as shown in FIG. 12C. InFIGS. 12A, 12B, and 12C, the valve lift curve of the cylinders of thefirst cylinder group is shown by the broken line, whereas the valve liftcurve of the cylinders of the second cylinder group is shown by thesolid line.

When switching between “operating” and “halt” of cylinders of a V-typeengine is performed by the valve drive system according to an embodimentof the present invention, it is preferable to provide only one driveunit in each bank in order to achieve cost reductions.

In order to perform switching between “operating” and “halt” of thecylinders of the first cylinder group, switching between “operating” and“halt” of the first cylinder #1 and the seventh cylinder #7 located atboth ends in a direction in which the cylinders are arranged is requiredto be performed by one drive unit in bank 1. Switching between“operating” and “halt” of the fourth cylinder #4 and the sixth cylinder#6 that are adjacent to each other is required to be performed by onedrive unit 13 in bank 2.

Switching between “operating” and “halt” of the fourth cylinder #4 andthe sixth cylinder #6 in bank 2 can be performed by the same arrangementas the valve drive system 1 shown in FIGS. 1 to 9B because thesecylinders are adjacent to each other.

However, switching between “operating” and “halt” of the first cylinder#1 and the seventh cylinder #7 in bank 1 cannot be performed by thevalve drive system 1 shown in the above embodiment. The reason is thatother cylinders exist between the first cylinder #1 and the seventhcylinder #7. This applies to a case in which switching between“operating” and “halt” of the cylinders of the second cylinder group isperformed. In other words, as shown in FIG. 12C, switching between“operating” and “halt” of the eighth cylinder #8 and the second cylinder#2 of the second cylinder group cannot be performed by the valve drivesystem 1 shown in FIGS. 1 to 9B.

Therefore, the present embodiment provides an arrangement in which athrust force is transmitted by taking advantage of the rocker shaft 17as shown in FIGS. 13A and 13B.

In the present embodiment, the rocker shaft 17 includes outer rockershafts 21A to 21D and an inner rocker shaft 23 that penetrates the axialcenter portions of the outer rocker shafts 21A to 21D. The outer rockershaft 21A is used for the first cylinder #1, and the slider 15 of thedrive unit 13 is connected to the outer rocker shaft 21A. The outerrocker shaft 21B is used for the third cylinder #3. The outer rockershaft 21C is used for the fifth cylinder #5. The outer rocker shaft 21Dis used for the seventh cylinder #7.

The outer rocker shaft 21A for the first cylinder #1 is movable in theaxial direction together with the two rocker arms 4 of the firstcylinder #1. The outer rocker shaft 21D for the seventh cylinder #7 ismovable in the axial direction together with the two rocker arms 4 ofthe seventh cylinder #7. The outer rocker shaft 21B for the thirdcylinder #3 and the outer rocker shaft 21C for the fifth cylinder #5 areattached to the cylinder head 5 such that these shafts cannot move intheir axial directions.

The inner rocker shaft 23 is connected to the outer rocker shaft 21A forthe first cylinder #1 and to the outer rocker shaft 21D for the seventhcylinder #7 so as not to be moved in its axial direction. The innerrocker shaft 23 movably penetrates the axial center portions of theouter rocker shaft 21B for the third cylinder #3 and the axial centerpart of the outer rocker shaft 21C for the fifth cylinder #5.

In other words, the rocker shaft 17 is arranged so that a thrust forceis transmitted to the outer rocker shaft 21D for the seventh cylinder #7through the inner rocker shaft 23 from the outer rocker shaft 21A forthe first cylinder #1 to which the slider 15 of the drive unit 13 isconnected.

In the present embodiment, the outer rocker shaft 21A for the firstcylinder #1, the outer rocker shaft 21D for the seventh cylinder #7, andthe inner rocker shaft 23 compose the first rocker shaft. The firstrocker shaft is arranged so as to move in the axial direction togetherwith the slider 15 and the rocker arm 4. Additionally, in the presentembodiment, the outer rocker shaft 21B for the third cylinder #3 and theouter rocker shaft 21C for the fifth cylinder #5 compose the secondrocker shaft. The second rocker shaft is arranged so as to be locatedcoaxially with the first rocker shaft and so as to be relatively movablein the axial direction with respect to the first rocker shaft.

Cams 82 and 85 of the first and seventh cylinders #1 and #7 includeactive cams 82 a and 85 a and dormant cams 82 b and 85 b, respectively.

The active cams 82 a and 85 a are arranged so as to have the same shapethat differs in phase from the cam 83 of the third cylinder #3 and thecam 84 of the fourth cylinder #4.

The dormant cams 82 b and 85 b are each formed in a disk shape that hasthe same diameter as the basic circle portion of each of the active cams82 a and 85 a in which the amount of valve lift becomes 0. In otherwords, the dormant cams 82 b and 85 b are arranged so that the amount ofvalve lift becomes 0 without depending on the rotation angle (phase) ofthe crankshaft 3.

In the present embodiment, the first cylinder #1 and the seventhcylinder #7 of bank 1 can be switched by the drive unit 13 from anoperating state to a dormant state, and the fourth cylinder #4 and thesixth cylinder #6 of bank 2 can be switched by a drive unit (not shown)from an operating state to a dormant state. As a result, the V-typeeight-cylinder engine can be operated substantially as a V-typefour-cylinder engine. Even if an arrangement in which switching between“operating” and “halt” is performed is employed in the cylinders of thesecond cylinder group, the same effect can be obtained.

In the present embodiment, the rocker arm 4 for the first cylinder #1and the rocker arm 4 for the seventh cylinder #7 that are supported bythe first rocker shaft receive a thrust force transmitted from the driveunit 13. On the other hand, the rocker arm 4 for the third cylinder #3and the rocker arm 4 for the fifth cylinder #5 that are supported by thesecond rocker shaft do not receive the thrust force transmittedtherefrom. Therefore, according to the present embodiment, the degree offreedom concerning the choice of cylinders that perform switchingbetween cams becomes high in the multi-cylinder engine.

In other words, in the valve drive system 81 according to the presentembodiment, the rocker arms 4 of the plurality of cylinders that are notadjacent to each other can be driven by the single drive unit 13.

Further, in the present embodiment, the rocker shaft 17 has a dualstructure, and therefore the rigidity of the rocker shaft 17 isheightened. Therefore, the operation of the valve-driving cams 82 to 84is accurately transmitted to the intake valve or to the exhaust valvethrough the rocker arm 4.

Fourth Embodiment

The holding mechanism can be arranged as shown in FIG. 14 and FIG. 15.In these figures, the same reference numeral is given to the same orequivalent member as in FIG. 1 to FIG. 9B, and a detailed description ofthe same or equivalent member is omitted.

The holding mechanism 35 according to the present embodiment is arrangedby using the rocker shaft 17.

The holding mechanism 35 shown in FIG. 14 includes two dents 91 formedon the outer peripheral surface of the outer rocker shaft 21 and a ball92 that can enter and leave the dents 91. The outer rocker shaft 21 isused to connect the slider 15 (see FIG. 1 and so forth) of the driveunit 13 and the rocker arm 4 together.

In the present embodiment, each dent 91 is an annular groove that isformed on the outer peripheral surface of the outer rocker shaft 21 andthat extends in the circumferential direction.

The dents 91 are spaced out at predetermined intervals (pitches) in theaxial direction of the outer rocker shaft 21. The interval is equal tothe formation interval (pitch) between two cams that are switched by thevalve drive system according to one embodiment of the present invention.These two cams may be a pair of low-speed cam 11 and high-speed cam 12or may be a pair of active cam and dormant cam.

The ball 92 is movably inserted in a hole 93 defined by the cylinderhead 5. The ball 92 is pressed against the dent 91 by a compression coilspring 94 inserted in the hole 93. A bolt 95 to press the compressioncoil spring 94 against the ball 92 is screwed into the hole 93. The ball92 is an in-and-out member arranged so as to enter and leave the dent 91and so as to be fitted into the dent 91.

The holding mechanism 35 shown in FIG. 15 includes two dents 96 formedon the inner peripheral surface of the outer rocker shaft 21 and a ring97 formed in a shape that can enter and leave these dents 96. Each dent96 includes an annular groove that is formed on the inner peripheralsurface of the outer rocker shaft 21 and that extends in thecircumferential direction. The dents 96 are spaced out at predeterminedintervals (pitches) in the axial direction of the outer rocker shaft 21.The interval is equal to the formation interval (pitch) between two camsthat are switched by the valve drive system according to the presentinvention. These cams may be a pair of low-speed cam 11 and high-speedcam 12 or may be a pair of active cam and dormant cam.

The ring 97 is made of an elastic body. A rubber or a spring can be usedas the elastic body. The ring 97 is contained in an annular groove 98 ofthe inner rocker shaft 23 in a state of protruding from the outerperipheral surface of the inner rocker shaft 23. The ring 97 is anin-and-out member arranged so as to enter and leave the dent 96 and soas to be fitted into the dent 96.

The in-and-out member (the ball 92 or the ring 97) of the holdingmechanism 35 shown in FIG. 14 and FIG. 15 restrains the outer rockershaft 21 from moving in the axial direction. In the holding mechanism 35of FIG. 14, when a thrust force in the axial direction from the slider15 is applied to the outer rocker shaft 21, the ball 92 goes out fromthe dent 91 by the compression of the compression coil spring 94 byelastic deformation. In the holding mechanism 35 of FIG. 15, when athrust force in the axial direction from the slider 15 is applied to theouter rocker shaft 21, the ring 96 is elastically deformed, and goes outfrom the dent 96.

In other words, when the thrust force is applied to the outer rockershaft 21, the in-and-out member (the ball 92, the ring 97) goes out fromone of the dents 91 and 96 by the elastic deformation of the elasticmember (the compression coil spring 94, the ring 97). The in-and-outmember comes off from one of the dents 91 and 96, and, as a result, theouter rocker shaft 21 moves in the axial direction together with therocker arm 4, and switching between the cams is performed. Aftercompleting the switching therebetween, the in-and-out member is fittedinto the other one of the dents 91 and 96, and the outer rocker shaft 21is again restrained from moving in the axial direction.

Therefore, according to the present embodiment, a load in the axialdirection is not needed to restrain the slider 15 that has moved frommoving in the axial direction, and therefore a slide loss can berestricted. Therefore, according to the present embodiment, the powerloss of the engine is reduced.

If the holding mechanism 35 having the arrangement shown in FIG. 14 orFIG. 15 is used, the positioning groove 58 is not required to bedisposed on the camshaft 3. In this case, the drive units 13 and 14 canbe arranged as shown in FIG. 16. In FIG. 16, the same reference numeralis given to the same or equivalent member as in FIG. 1 to FIG. 9B, and adetailed description of the same or equivalent member is omitted.

The switching cam 31 for switching between the first cam mechanism 32and the second cam mechanism 33 shown in FIG. 16 is formed of only onecam groove 36 that has a predetermined depth in the radial direction ofthe camshaft 3. The cam groove 36 includes a wide linear part 37, anarrow linear part 38, and a tapered part 39 that connects these linearparts together. The wide linear part 37 has a pair of side walls 37 aand 37 a along the circumferential direction of the camshaft 3 and apartially cylindrical bottom surface 37 b formed between the side walls37 a and 37 a. The narrow linear part 38 has a pair of side walls 38 aand 38 a along the circumferential direction of the camshaft 3 and apartially cylindrical bottom surface 38 b formed between the side walls38 a and 38 a. The tapered part 39 has a pair of inclined side walls 39a and 39 a that are inclined in mutually opposite directions withrespect to the axial direction of the camshaft 3 and a partiallycylindrical bottom surface 39 b formed between the inclined side walls39 a and 39 a. The inclined side wall 39 a smoothly connects the sidewall 37 a of the wide linear part 37 and the side wall 38 a of thenarrow linear part 38 together. The cam followers 44 and 45 are guidedfrom the side wall 37 a of the wide linear part 37 to the side wall 38 aof the narrow linear part 38 through the inclined side wall 39 a of thetapered part 39, and, as a result, the slider 15 moves in the axialdirection of the camshaft 3. The slider 15 is supported by both ends ofthe large diameter portion 16 of the camshaft 3 so as to be movable inthe axial direction of the camshaft 3.

The side wall 37 a of the wide linear part 37 corresponds to the outerside wall of the linear part 59 in the arrangement of FIG. 6. The narrowlinear part 38 corresponds to a part of the positioning groove 58 in thearrangement of FIG. 6 that excludes a range in the circumferentialdirection in which the movement groove 57 is formed. The side wall 39 aof the tapered part 39 corresponds to the outer side wall of theinclined part 56 in the arrangement of FIG. 6.

Fifth Embodiment

A hydraulic power source can be used as the power source of the actuator46 as shown in FIG. 17. In FIG. 17, the same reference numeral is givento the same or equivalent member as in FIG. 1 to FIG. 9B, and a detaileddescription of the same or equivalent member is omitted.

The actuator body 60 shown in FIG. 17 is composed of cylindrical firstand second plungers 60 a and 60 b that face the lifters 47 and 48,respectively, and a hydraulic cylinder 101 that drives these plungers 60a and 60 b.

The hydraulic cylinder 101 is formed by fitting pistons 104 into twocylinder holes 102 and 103 defined in the cylinder head 5, respectively.The cylinder holes 102 and 103 are connected to a hydraulic controlvalve 107 through hydraulic passages 105 and 106, respectively. Thehydraulic control valve connects either of the two cylinder holes 102and 103 to a hydraulic source 108.

The two pistons 104 face the first and second plungers 60 a and 60 b,respectively.

The hydraulic source 108 that supplies oil pressure to the hydrauliccylinder 101 includes, for example, a hydraulic pump 109 constructed todischarge oil while rotating together with the crankshaft of the engine.Therefore, the power source of the actuator 46 is never lost during theoperation of the engine.

Therefore, according to the present embodiment, it is possible toprovide a valve drive system of an engine having a high operationalreliability.

Additionally, a conventionally well-known existing one can be used asthe hydraulic control valve 107 that controls the operation of theactuator 46. Therefore, a valve drive system according to the presentembodiment can be produced without causing a great increase in cost.

Sixth Embodiment

The slider 15 of the drive unit 13 can also be arranged so as to besupported by the rocker shaft 17 as shown in FIG. 18. In FIG. 18, thesame reference numeral is given to the same or equivalent member as inFIG. 1 to FIG. 9B, and a detailed description of the same or equivalentmember is omitted.

The slider 15 of the drive unit 13 shown in FIG. 18 is supported by theouter rocker shaft 21 in a state of being unable to relatively move inthe axial direction with respect to the rocker shaft 21.

The slider 15 has a guide portion 111 that comes into contact with thelarge diameter portion 16 of the camshaft 3 from the outside in theradial direction. The guide portion 111 is arranged so as to prevent theslider 15 from rotating by the rotation of the camshaft 3. The guideportion 111 is formed in a circular-arc shape along the outer peripheralsurface of the large diameter portion 16.

Further, in the arrangement according to the present embodiment as well,the same effect as in the embodiments shown in FIG. 1 to 9B can beattained.

Although an example in which the present invention is applied to amulti-cylinder engine has been described in the above embodiments, thepresent invention is applicable to a single-cylinder engine.Additionally, although an example in which switching between two cams isperformed has been described in the above embodiments, the number ofcams to be switched is not limited to two, and switching among three ormore cams can be performed in the valve drive system according to thepresent invention. For example, in a case that switching among threecams is desired, the number of switching cams 31 and the number of camfollowers are increased, accordingly.

Although the embodiments of the present invention have been described indetail as above, these are merely specific examples used to clarify thetechnical contents of the present invention, and the present inventionshould not be understood as being limited thereto, and the scope of thepresent invention is to be determined solely by the appended claims.

The present application corresponds to Japanese Patent Application No.2009-232203 filed in the Japan Patent Office on Oct. 6, 2009, and theentire disclosure of the application is incorporated herein byreference.

DESCRIPTION OF SIGNS

-   -   1 . . . Valve drive system, 3 . . . Camshaft, 5 . . . Cylinder        head, 7 . . . Head cover, 11 . . . Low-speed cam, 12 . . .        High-speed cam, 13, 14 . . . Drive unit, 15 . . . Slider, 16 . .        . Large diameter portion, 17 . . . Rocker shaft, 19 . . .        Presser, 21 . . . Outer rocker shaft, 23 . . . Inner rocker        shaft, 31 . . . Switching cam, 32 . . . First cam mechanism, 33        . . . Second cam mechanism, 34 . . . Driving mechanism, 35 . . .        Holding mechanism, 44 . . . First cam follower, 45 . . . Second        cam follower, 46 . . . Actuator, 49 . . . Spring member, 56 . .        . Inclined part, 57 . . . Movement groove, 58 . . . Positioning        groove, 59 . . . Linear part, 60 . . . Actuator body, 60 c . . .        Solenoid, 63, 72 a to 75 a, 82 a . . . Active cam, 64, 72 b to        75 b, 82 b . . . Dormant cam, 101 . . . Hydraulic cylinder

1. A valve drive system of an engine, the valve drive system comprising:a camshaft that is supported by a cylinder head of the engine and onwhich a plurality of cams having different valve lift characteristicsare formed at a predetermined formation interval; a rocker shaftsupported by the cylinder head in parallel with the camshaft; a rockerarm that is disposed between one of the plurality of cams and an intakevalve or an exhaust valve, that is swingably supported by the rockershaft, that is arranged so as to be movable in an axial direction of therocker shaft, and in which a presser with respect to the intake valve orthe exhaust valve extends in the axial direction with a length greaterthan the formation interval between the plurality of cams; and a driveunit that moves the rocker arm toward one side or toward an oppositeside in the axial direction by the formation interval between theplurality of cams.
 2. The valve drive system of the engine according toclaim 1, wherein the drive unit is supported by a part differing fromthe rocker arm.
 3. The valve drive system of the engine according toclaim 1, wherein the driving mechanism includes a switching cam arrangedintegrally with the camshaft, and is arranged so as to generate a thrustforce to move the rocker arm in the axial direction when amounts ofvalve lifts of the plurality of cams are 0 while using the switchingcam.
 4. The valve drive system of the engine according to claim 1,wherein the drive unit includes: a driving mechanism that transformsrotation of the camshaft into a thrust force toward one side or towardan opposite side in the axial direction of the camshaft; a slider thatis driven by the driving mechanism to move in the axial direction of thecamshaft; a connecting mechanism that connects the slider and the rockerarm; and a holding mechanism that holds the slider at a position towhich the slider has moved, the driving mechanism including: a first cammechanism that moves the slider toward one side in the axial directionwhen amounts of valve lifts of the plurality of cams are 0; a second cammechanism that moves the slider toward an opposite side in the axialdirection when amounts of valve lifts of the plurality of cams are 0;and an actuator that performs switching between “use” and “nonuse” ofthe first and second cam mechanisms, and wherein a movement distance ofthe slider moved by the first and second cam mechanisms is set to beequal to the formation interval between the plurality of cams or is setto be a value close to the formation interval between the plurality ofcams.
 5. The valve drive system of the engine according to claim 4,wherein each of the first cam mechanism and the second cam mechanismincludes: a switching cam including a cam groove that has apredetermined depth in a radial direction of the camshaft and thatextends in a circumferential direction and in the axial direction of thecamshaft; and a cam follower arranged so as to be guided by theswitching cam, wherein the actuator is arranged so as to reciprocate thecam followers of the first and second cam mechanisms between a useposition at which the cam followers are guided while being in contactwith the switching cam and a nonuse position at which the cam followersare apart from the switching cam outwardly in the radial direction,wherein the slider is supported by a portion of the camshaft at whichthe switching cam is formed relatively rotatably with respect to thecamshaft, and is held such that rotation around the camshaft isrestrained by the connecting mechanism, and wherein the cam followers ofthe first and second cam mechanisms are movably supported by the slider.6. The valve drive system of the engine according to claim 5, whereinthe connecting mechanism is arranged so as to transmit a thrust forcefrom the slider to the rocker arm through the rocker shaft.
 7. The valvedrive system of the engine according to claim 4, wherein the switchingcam includes: a movement groove that has an inclined part used to movethe slider in the axial direction; and an annular positioning groovethat extends in the circumferential direction of the camshaft at a sameposition in the axial direction as a terminal of the inclined part, andwherein the holding mechanism includes the positioning groove and thecam follower.
 8. The valve drive system of the engine according to claim7, wherein the positioning groove of the first cam mechanism and thepositioning groove of the second cam mechanism are formed at a sameposition in the axial direction.
 9. The valve drive system of the engineaccording to claim 7, wherein a depth of the positioning groove is equalto or greater than a depth of the movement groove.
 10. The valve drivesystem of the engine according to claim 5, wherein the actuatorincludes: a lifter for each cam follower, the lifter being attached to afront end of the cam follower and being supported so as to enter andleave the slider; a spring member that presses the lifter in a directionin which the lifter leaves the slider; and an actuator body that facesthe lifter, the actuator body being supported by a cylinder head or ahead cover of the engine, the actuator body including a plurality ofplungers that proceed to and recede from the lifter.
 11. The valve drivesystem of the engine according to claim 6, wherein the rocker shaftincludes: a first rocker shaft that moves in an axial direction togetherwith the slider and the rocker arm; and a second rocker shaft arrangedso as to be located coaxially with the first rocker shaft and so as tobe relatively movable in the axial direction with respect to the firstrocker shaft, wherein the first rocker shaft is joined to the rockerarms corresponding to a plurality of cylinders of the engine so that thethrust force is transmitted thereto, and wherein the first cam mechanismand the second cam mechanism are arranged so as to generate a thrustforce by which the slider is moved when amounts of valve lifts become 0in the plurality of cylinders.
 12. The valve drive system of the engineaccording to claim 4, wherein the rocker shaft includes: an outer rockershaft that is shaped like a pipe and to which the rocker arm isattached; and an inner rocker shaft that is movably fitted to an insideof the outer rocker shaft, wherein the connecting mechanism is arrangedso as to transmit a thrust force from the slider to the rocker armthrough the outer rocker shaft, and wherein the holding mechanismincludes: a dent formed on an outer surface or on an inner surface ofthe outer rocker shaft; and an in-and-out member that is arranged so asto be able to go in and out of the dent and that is arranged so as to bepressed against the dent by elasticity.
 13. The valve drive system ofthe engine according to claim 4, wherein a power source of the actuatoris an electrically-operated driving source.
 14. The valve drive systemof the engine according to claim 4, wherein a power source of theactuator is a hydraulic driving source.
 15. The valve drive system ofthe engine according to claim 13, wherein the actuator is arranged suchthat, in an OFF state, one of the first and second cam mechanismsreaches a use state, and a remaining one thereof reaches a nonuse state.